1. Field of the Invention
The present invention relates to a device and method for separating oil in a rotary compressor system and, specifically, to a filtration vessel which acts as a lube oil separation coalescer to eliminate liquid particles from a gas stream utilizing a rotary screw compressor.
2. Description of the Prior Art
Helical lobe rotary compressors, commonly referred to as “screw compressors” are becoming more and more popular in the air compressor, refrigeration and natural gas processing industries. These screw type processors utilize rotors or screws to push or compress gas to a higher pressure. Commonly, two cylindrical rotors are mounted on separate shafts inside a hollow, double-barreled casing. The sidewalls of the compressor casing form two parallel, overlapping cylinders which house the rotors in side by side fashion with their shafts generally parallel to the ground. Helically extending lobes and grooves on the exterior surface of the rotors mesh with one another during operation to form a series of chevron-shaped gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor gas inlet with the gaps being continuously reduced in volume as the rotors turn in order to compress the gas toward a discharge port at the opposite end of the casing. The screw compressor rotors intermesh within the housing and rotate in opposite directions in synchronization. As a result, gas is swept through the housing from the intake manifold at one end of the housing to an output manifold at the opposite end.
The rotor shafts are typically supported at either end of the casing by lubricated bearings or seals that receive a constant supply of lubricant from a lubricant circulation system. The lubricants are typically oil-based liquids, commonly referred to as “lube oils” in the industry. These oils are typically “synthetic” oils which are more expensive than typical engine motor oils. The screw compressors of the type under consideration also include a means for recirculating the lube oil through the inside of the compressor casing. The injection of Tube oil directly in to the gas stream helps to cool and lubricate the rotors, blocks gas leakage between and around the rotors, inhibits corrosion and minimizes the screw compressor noise level.
As a result of the lubrication system utilized, a typical oil flooded screw compressor discharges a high pressure and high temperature stream which is a mixture of the lube oil and compressed gas. The oil and any other liquid or solid contaminants must be separated from the high pressure gas stream. This particular application is challenging due to the large concentration of lube oil as free liquid and aerosols in the fluid stream. Standard gas coalescers handle a relatively small amount of liquid whereas screw compressor lube oil separation coalescers can be designed to handle lube oil rates of up to 200 GPM. Most of the prior art designs have relied upon gravity separation in the inlet or storage sections of the filtration vessel and have utilized standard gas industry coalescer elements for the second stage, aerosol and mist removal section of the vessel.
It is important that the lube oil separation coalescer be very efficient at removing lube oil. Excessive continual loss of lube oil becomes a costly maintenance item as most screw compressors utilize expensive synthetic oils. The synthetic lube oils also build up in the natural gas pipeline to form a difficult to remove contaminant that commonly causes fouling of natural gas processing equipment.